EWSR1 and Ewing sarcoma: Detailed in this review, in the absence of an inducible in vitro or in vivo model of transformation in Ewing sarcoma, multipotent mesenchymal stromal cells [a.k.a. mesenchymal stem cells (MSCs)] have emerged as not only the most likely candidate cell of origin, but provide a workable cellular platform to which EWS/FLI-mediated transformation can be more thoroughly characterized and understood.